Cooling, operational and waste water from the daily operation of nuclear power plants and fuel rod holding tanks is contaminated with a number of radioactive isotopes which are present in the water in very low concentrations but which nonetheless are highly radioactive and toxic to human life. Since the volumes of contaminated water are extremely large, it is neither possible nor economically feasible to store away this water permanently. Instead, the water must be either re-used or released back into the environment. Safe disposal or re-use of the contaminated water can only be conducted if a sufficient quantity of radioactive isotopes are removed from it to reach permissible levels. The method of ion exchange is the most promising and most in use today because of the large volume reduction of waste material.
The radioactive isotopes present in contaminated nuclear reactor water occur as cations, anions or solids, and any complete disposal system must handle all three species. Radioactive isotopes present in typical reactor cooling water are listed in Table 1. This water also generally contains non-radioactive isotopes of boron (400 ppm), sodium (150 ppm) and chlorine (5 ppm).
The most dangerous cations from the stand point of high concentrations and long-half lives are Cs, Co and Sr. These are removed with the other dissolved cations by a cation exchanger.
In copending applications U.S. Ser. No. 959,222 filed Nov. 9, 1978 and U.S. Ser. No. 39,595, filed May 16, 1979, each entitled Fixation By Ion Exchange of Toxic Materials In a Glass Matrix and each by C. J. Simmons, J. H. Simmons, P. B. Macedo, and T. A. Litovitz the removal of radioactive cations from reactor cooling systems using a porous glass cation exchanger is disclosed.
The anions present in solution consist primarily of I.sup.131 which has a half-life of 8 days but which poses a significant threat to life due to its affinity for and high reconcentration in animal and human metabolic processes. Most other anion isotopes are also short-lived and due to this rapid decay, they have a stronger tendency to damage their ion-exchange hosts than do the cation isotopes. After 3 months, the majority of the non-metal anions have generally decayed to stable isotopes, however many of the longer-lived metal isotopes form anionic complexes such as chromates, cerates, and molybdates, which remain radioactive for longer time periods. For example, the half-lives of Cr.sup.51, Ce.sup.144, and Te.sup.99 are 26 days, 290 days and 200,000 years respectively.
Today, organic anion resins are used in nuclear reactors, however, they are readily decomposed by radioactivity, they cannot be dried, they are not compatible for use in mixed beds with the new types of glass cation exchangers coming on the market, and they cannot be put into a long-term chemically stable form, thus causing a serious danger to the environment through premature release of the radioactive isotopes. The present invention is directed to the disposal of both poisonous radioactive anions such as radioactive iodide, chromate, molybdate, cerate and technetium, and non-poisonous or non-radioactive isotopes. The non-poisonous anions must be removed from nuclear waste streams to protect parts which are in contact with the streams. Exemplary parts are fuel elements, tubing, heat-exchangers, reactor vessels. Chloride is the predominant non-radioactive anion which must be removed.
The present invention discloses the use of an especially prepared porous glass medium as an anion exchanger. The glass anion exchanger is far superior to the organic exchangers available today for the following reasons. It is insensitive to radiation (such as from short-lived isotopes). It is compatible with the new glass cation exchangers and can be used in mixed-bed exchanger media with them. It can be dried, thus reducing the dissemination of radioactive isotopes after use. It can be heated to permanently fixate radioactive isotopes within its pores and produce a long-term chemically stable form which will resist premature dissemination of radioactive isotopes into the environment. Finally, as can be seen from Table 1, and Table 4, there are a large number of radioactive isotopes which occur as solids. These solids are dangerous and have long half-lives such as Co.sup.60 with 5 years. These solids do not chemically bond to the ion exchange media, however they remain entrapped between the grains of the ion exchanger by simple filtering action. As a result, they are effectively removed from the water by the ion exchange media. If the media are organic resins, they are encased in cement or bitumen, neither of which have a good long-term chemical stability, and the filtered solids are the first to be released to the environment, thus causing a serious health hazard. If the media are the anion-exchanged glasses disclosed here, it is possible to heat them to moderate temperatures and cause sintering of the ion exchange powder, thus permanently fixating these toxic, radioactive solids in the glass structure and effectively isolating them from the environment.
TABLE 1 ______________________________________ Typical radioactive isotopes in reactor cooling water typical concentration .mu.Ci/ml chemical form ______________________________________ .gamma.-emitters Co-57 1.3 .times. 10.sup.-5 cation, solid Cr-51 3.5 .times. 10.sup.-4 anion I-131 2.0 .times. 10.sup.-4 anion Cs-134 1.1 .times. 10.sup.-3 cation Cs-137 1.8 .times. 10.sup.-3 cation Zr-95 7.1 .times. 10.sup.-5 solid Nb-95 8.6 .times. 10.sup.-5 solid Co-58 6.7 .times. 10.sup.-3 cation, solid Fe-59 2.6 .times. 10.sup.-4 cation, solid Ba--La-140 1.2 .times. 10.sup.-5 cation Cs-136 3.1 .times. 10.sup.-5 cation Mn-54 6.9 .times. 10.sup.-4 cation, solid Co-60 3.5 .times. 10.sup.-3 cation, solid Non .gamma.-emitters Sr-90 cation Y-90 cation H-3 cation, anion C-14 cation, anion, solid Other .gamma.-emitter iso- topes found in trace amounts Np-239 cation Ce-144 anion, solid Ce-139 anion, solid Sn-113 anion Zn-69M cation, solid Co-138 cation W-187 solid I-133 anion As-76 anion Cs-134 cation Nb-97 solid Mo-99 anion Zr-97 solid I-132 anion I-134 anion Ag-110M solid Zu-65 cation, solid Na-22 cation Cu-64 cation, anion, solid Na-24 cation K-40 cation Ni-65 cation, solid K-42 cation Cl-38 anion Mn-56 cation, solid Rb-88 cation I-135 anion ______________________________________
The two most popular types of commercial reactors, both of which produce low level wastes, are the Boiling Water Reactor (B.W.R.) and the Pressurized Water Reactor (P.W.R.). In a typical Pressurized Water Reactor (P.W.R.), pressurized light water circulates through the reactor core (heat source) to an external heat sink (steam generator). In the steam generator, where primary and secondary fluids are separated by impervious surfaces to prevent contamination, heat is transferred from the pressurized primary coolant to secondary coolant water to form steam for driving turbines to generate electricity. In a typical Boiling Water Reactor (B.W.R.), light water circulates through the reactor core (heat source) where it boils to form steam that passes to an external heat sink (turbine and condenser). In both reactor types, the primary coolant from the heat sink is purified and recycled to the heat source.
The primary coolant and dissolved impurities are activated by neutron interactions. Materials enter the primary coolant through corrosion of the fuel elements, reactor vessel, piping, and equipment. Activation of these corrosion products adds radioactive nuclides to the primary coolant. Corrosion inhibitors, such as lithium, are added to the reactor water. A chemical shim, boron, is added to the primary coolant of most P.W.R.'s for reactivity control. These chemicals are activated and add radionuclides to the primary coolant. Fission products diffuse or leak from fuel elements and add nuclides to the primary coolant. Radioactive materials from all these sources are transported around the system and appear in other parts of the plant through leaks and vents as well as in the effluent streams from processes used to treat the primary coolant. Gaseous and liquid radioactive wastes (radwaste) are processed within the plant to reduce the radioactive nuclides that will be released to the atmosphere and to bodies of water under controlled and monitored conditions in accordance with federal regulations.
The principal methods or unit operations used in the treatment of liquid radwaste at nuclear power plants are filtration, ion exchange, and evaporation.
Liquid radwastes in a P.W.R. are generally segregated into five categories according to their physical and chemical properties as follows:
a. Clean Waste includes liquids which are primarily controlled releases and leaks from the primary coolant loop and associated equipment. These are liquids of low solids content which are treated in the reactor coolant treatment system. PA1 b. Dirty or Miscellaneous Waste includes liquids which are collected fom the containment building, auxiliary building, and chemical laboratory; regeneration solutions from ion-exchange beds; and solutions of high electrical conductivity and high solids content from miscellaneous sources. PA1 c. Steam Generator Blowdown Waste is condensate from the steam that is removed (blowdown) periodically to prevent excessive solids buildup. PA1 d. Turbine Building Drain Waste is leakage from the secondary system that is collected in the turbine building floor sump. PA1 e. Detergent Waste includes liquids from the laundry, personnel decontamination showers, and equipment decontamination. PA1 a. High-Purity Waste includes liquids of low electrical conductivity (&lt;50 .mu.mho/cm) and low solids content, i.e., reactor coolant water that has leaked from the primary reactor system equipment, the drywell floor drain, condensate demineralizer backwash, and other sources of high-quality water. PA1 b. Low-Purity Waste includes liquids of electrical conductivity in excess of 50 .mu.mho/cm and generally less than 100 .mu.mho/cm; i.e, primarily water from floor drains. PA1 c. Chemical Waste includes solutions of caustic and sulfuric acid which are used to regenerate ion exchange resins as well as solutions from laboratory drains and equipment decontamination. PA1 d. Detergent Waste includes liquids from the laundry and personnel decontamination showers.
Liquid radwastes in a B.W.R. are generally segregated into four categories according to their physical and chemical properties as follows:
The liquid radwastes from both types of reactors are highly dilute solutions of radioactive cations, anions and other dissolved radioactive materials as well as undissolved radioactive particles or finely divided solids.
A practical process for disposing of radioactive materials in a dry solids form having high resistance to leaching and other forms of chemical attack would not only be suitable for the disposal of radioactive nuclear wastes, but also for the fabrication of radioactive sources useful in industry, medicine, and in the laboratory.
Heretofore, there did not exist any practical foolproof means for the safe disposal, storage and immobilization of pernicious radioactive waste material. Present day storage containers do not provide sufficient isolation and immobilization of such radioactive material, sufficient long-term resistance to chemical attack by the surroundings, and sufficient stability at high temperatures.
Currently low level radioactive waste, that is radioactive waste generated at reactor sites, is disposed of in the following manner:
(A) The dead ion exchange resin containing radioactive waste is mixed with cement or bitumen and cast in forty gallon barrels.
(B) The bottoms from evaporators which contain the radioactive contaminated boric acid and the solutions used to regenerate the ion exchange columns are mixed with cement powder or bitumen and cast in forty gallon barrels.
(C) The filters containing particulate forms of radioactive waste are usually encased in cement or bitumen in barrels.
These cement or bitumen barrels are transported to low level radioactive waste sites and buried six to twenty feet deep in the ground. At least one of the sites is in the United States eastern states and exposed to substantial rainfall. In Europe, these barrels are buried at sea. In both cases water will first corrode the metal then the cement and will relatively quickly expose the radioactive ions for leaching into the ground water or sea water. Because the U.S. burials are only a few feet deep, the contaminated water can readily intermix with streams, lakes and rivers, thus, entering the ecosphere. The rationale for this practice is the assumption that upon sufficient dilution the radioactivity becomes harmless.
Some of the most serious nuclear wastes are cesium and strontium which are biologically similar to sodium and calcium. They have thirty year half-lives indicating that they should be isolated from the ecosphere for at least three hundred years (ten half-lives). At Bikini, the experts assumed that dilution had made the island inhabitable after decades in which no atomic explosions were performed, yet when the population was returned to the island its health was deleteriously effected. It has since been realized that plants and animal life biologically reconcentrate these radioactive elements back up to dangerous levels.
Thus, the "safe" concentration of radioactive waste must be much lower than accepted values and a more durable substitute for cement is needed. In one aspect, the present invention presents a safe alternative to the cement-solidification of low level waste.
U.S. Pat. No. 3,640,888 teaches the production of neutron sources by encapsulating californium-252 in glass using the steps of packing an open-ended vitreous tube with a porous powder of quartz having a organic liquid ion exchange material sorbed thereon, passing an aqueous solution containing californium-252 cation through the powdered quartz, drying and heating the powdered quartz and tube in air to oxidize and volatilize the organic liquid ion exchange material resulting in the non-volatile oxide of californium-252, and then fusing the tip and powder contents to form a vitreous body containing the californium-252 oxide. The patent, however, does not disclose, teach or suggest the use of porous glass or silica having aminoorganosilyloxy groups bonded to silicon and/or having hydrous metal oxides bonded to silicon through divalent oxygen linkages wherein hydroxyl groups are exchanges for radioactive anions in aqueous solution nor does it disclose or suggest any method or technique for concentrating and safely disposing of radioactive wastes.
As will be apparent hereinafter from the various aspects of applicants' contributions to the art, there are provided novel methods to obtain novel compositions and articles for the containment of pernicious and dangerous radioactive materials over extraordinarily long periods of time. Unlike melting glass containment procedures, the methods of the invention need not involve any steps which would expose radioactive material to high temperatures, e.g., above about 900.degree. C., thereby eliminating the environmental hazard due to possible volatilization of radioactive material into the atmosphere.
Belgian Pat. No. 839,705, issued July 16, 1976 and German Offenlegungsschrift 2,611,495, published July 10, 1976 correspond substantially to U.S. Pat. No. 4,110,096, issued Aug. 29, 1978 to Pedro B. Macedo named as an inventor herein and Theodore A. Litovitz. These patents and Offenlegungsschrift contain essentially the same disclosures but there is no disclosure of porous glass forms having sufficient ion exchange capabilities to bind practical amounts of radioactive anions to the glass to thereby concentrate and contain said radioactive anions in the manner taught herein.
The presence of silica gel in the pores can be advantageous in this invention as providing more surface area and a higher proportion of silicon-bonded hydroxyl groups and ultimately higher amounts of organofunctionalsiloxy bonded hydroxyl groups or hydrated metal oxide groups for ion exchange with radioactive anions. U.S. Pat. No. 4,110,096 also discloses oxides or salts of heavy metals such as zirconium, lead, and thorium as dopants for the porous glass. The dopants are precipitated in the pores, the porous glass is washed in water or an acidic solution, dryed and sintered. However, there is no teaching or suggestion of binding a hydrous metal oxide to silicon of a porous glass or porous silica gel through divalent oxygen linkages prior to heating to sintering temperatures and thereafter reacting the resulting product with radioactive or toxic anions.
In an article by Amphlett et al., entitled, "Synthetic Inorganic Ion-Exchange Materials-II Hydrous Zirconium Oxide And Other Oxides," J. Inorg. Nucl. Chem., Vol. 6, pp. 236 to 245 (1958), hydrous oxides, such as hydrous zirconium oxide, are disclosed as anion exchangers in acid and neutral solution and as cation exchangers in alkaline solution. The Amphlett et al. article is herein incorporated by reference in its entirety. There is no teaching or suggestion in the Amphlett et al. article of binding the hydrous metal oxide to the silicon atoms of a porous glass or porous silica gel through divalent oxygen linkages and reacting the resulting product with radioactive or toxic anions.
U.S. Pat. No. 2,943,059 discloses porous glass ion exchange glass for removal of the radioactive ions cesium and strontium: anions are not specifically mentioned. The glass composition must contain at least 10% titanium dioxide, zirconium dioxide, or hafnium dioxide and at least 20% PO.sub.2.5 which combines in a unique manner with the above three oxides. The reference does not teach or render obvious a porous glass or silica gel having an SiO.sub.2 content of at least 82% by weight (SiO.sub.2 is an optional ingredient). The high silica content is needed in the present invention for obtaining a glass of high durability. Also, hydration of the hydrous metal oxide groups to form an anion exchange medium is not disclosed. Collapsing of the porous structure after ion exchange is not disclosed.
U.S. Pat. No. 3,843,341 teaches forming porous glass beads which may contain more than 96% by weight silica and impregnating them with various metal salts, including nitrates, followed by heat decomposing the metal salt to form the corresponding metal oxide, e.g., titanium dioxide and tin oxide. The porous products may contain greater than 96% by weight silica. The product is used as a catalyst and the hydrated form is not specified.
U.S. Pat. No. 3,923,688 teaches a high silica porous glass (at least 96% by weight SiO.sub.2) which can be used as a catalyst support, a filter, a cation exchanger, or an anion exchanger. When used as a catalyst support, zirconium oxide, which apparently has a catalytic effect and/or imparts thermal stability to the catalyst, is deposited within the pores of the glass by decomposition of zirconium nitrate. A porous glass having hydrous zirconium oxide bonded thereto is not disclosed as an anion exchanger. Production of a cation exchanger, however, is disclosed wherein sodium ions are placed on the surface of the porous glass. Conversion of the glass to an anion exchanger by attaching hydrous metal oxides to the glass surface is not disclosed. Also, treatment of the porous glass with an organosilane is disclosed. The product is disclosed as being also useful as a support for chromatographic separations. However, anion exchangers having hydrated aminoorganosilyloxy groups at the surface of the porous glass are not disclosed. Furthermore, removal of radioactive ions from aqueous radwaste solutions are not disclosed and there is no mention of collapsing the pores of the glass.
U.S. Pat. No. 4,025,667 discloses porous glass having a coating of zirconium oxide thereon. The zirconium oxide coating may be silanized with an organofunctionalsilane coupling agent. The organofunctional portion of the silane coupling agent is used to immobilize enzymes. Removal of radioactive ions from radwastes is not disclosed. In addition, the pores of the porous glass are not subsequently collapsed. The reference does not teach hydrating the zirconium oxide coated glass to form an anion exchange medium having ion exchangeable hydroxyl groups attached to the zirconium ion.
U.S. Pat. No. 3,969,261 discloses ion exchangers comprising porous silica gel beads, or other porous silica supports having a tertiary aminoalkylsilane bonded to oxygen of the silica gel to produce .tbd.SiOSi.tbd. bonds. These ion exchangers are made by reacting a (dialkylamino)alkoxysilane with the hydroxyl groups of a silica gel which can also contain hydroxides of titanium, zirconium and thorium. Neither removal of radioactive ions from radwastes nor subsequent collapsing the pores is disclosed.
U.S. Pat. No. 4,118,316 discloses high silica porous glass beads of carefully controlled pore size which are reacted with an aminoalkylsilane, such as gamma-aminopropyltriethoxysilane. The resulting product is then quaternarized with a hydrocarbon halide or tertiary amine to introduce quaternary ammonium moieties on the beads. The quaternary ammonium moieties are used to separate cation polymers into molecular weight fractions. Hydration of the amino group of the aminoalkylsilane to produce anion exchange groups is not disclosed. Neither removal of radioactive ions nor subsequent collapsing of the pores of the porous glass is disclosed.
The 1979-80 Pierce Handbook & General Catalog, pages 355-379 discloses controlled pore porous glass supports for chromatography and more specifically discloses silylated aminoalkyl controlled pore glass supports for solid phase sequencing of large peptide fragments. This reference, however, fails to disclose, teach or suggest the removal of radioactive anions from aqueous radwaste solutions or subsequent collapsing of the porous structure.
U.S. Pat. No. 3,709,833 discloses porous silica glass forms which can contain zirconium oxide and which are primarily useful as catalyst supports. This patent does not disclose, teach or suggest that the porous silica glass forms disclosed therein can be used as anion exchangers nor does it disclose the removal of radioactive anions from aqueous radwaste solutions or the containerizing or burying of same underground or underwater or the collapsing of the pores containing the radioactive anions.
U.S. Pat. No. 2,614,135 discloses porous silica gels treated with aminoorganosilane to produce a product suitable for removing oil from oil-polluted waters.
U.S. Pat. No. 2,990,243 teaches the removal of fission products and/or plutonium from solutions by adsorbing the fission product and/or plutonium on a titanated silica gel.
U.S. Pat. No. 2,893,824 also teaches a titanated silica gel.
However, none of these references teach or suggest using a porous silica gel having an interconnected porous structure and having organofunctionalsiloxy groups bonded to silicon of the silica gel and/or hydrous polyvalent metal oxide groups bonded to silicon of the silica gel through divalent oxygen linkages as a backfill for nuclear waste disposal sites.
None of the afore-mentioned prior art references disclose, teach or suggest the removal of radioactive anions from an aqueous radwaste through the use of a silica gel or a porous silica glass containing at least 82 mol percent silica and having hydrated organofunctionalsiloxy groups bonded to silicon of the glass or gel and/or a hydrated polyvalent metal oxide bonded to silicon of the glass or gel or deposited within the pores thereof. These references fail to disclose or suggest the containerizing and burial underground or underwater of the containerized silica gel or porous glass impregnated with radioactive anions internally bonded therein. The references also fail to disclose or suggest collapsing of the pores of the silica gel or porous glass to encase the radioactive anions therein to provide articles useful as radiation sources or suitable for burial. Furthermore, the references fail to disclose, teach or suggest the use of silica gel or porous glass containing the hydrated organofunctionalsiloxy groups and/or hydrated polyvalent metal oxides as backfill for stored nuclear waste materials. In addition, there is no disclosure or suggestion in any of these references of the ionic bonding of polyvalent metal oxides to porous silica glass or silica gel by first exchanging the protons of the silicon-bonded hydroxyls with an alkali metal, e.g., sodium, or ammonium, cations followed by replacement of the alkali metal or ammonium cations with the polyvalent metal cations.